1. Field of the Invention
This invention relates to a system and a method to enable widespread adoption of geothermal energy and more particularly this invention relates to a system and a method for facilitating energy transfer from a geothermal field to an existing HVAC system of a building with minimal retrofit, thereby enhancing or retro fitting existing conventional HVAC systems with minimal interference of daily activity within the building.
2. Background of the Invention
Geothermal energy is an alternative energy source existing under ground. The goal for geothermal energy use is to utilize the typical midrange constant temperatures of 52 to 54° F. found beneath the earth's surface to help heat or cool a structure in winter or summer, respectively.
Conventional geothermal well fields are drilled with wells having uniform depths and approximately 4-6 inch diameters. Field depth may vary from 75 feet (or less) to 600 feet (or more). After each well is drilled, a high density geothermal loop is then inserted to the bottom of each well.
Once the loop is in place within the well, the well is pressure-grouted, starting from the bottom of the well up to its opening at the surface of the earth. The grouting both improves thermal conductivity of the loop with the surrounding formation, and seals the well bore to prevent contamination of the surrounding geological features.
Efficiency dictates that thermal conduction of underground temperatures to the thermal conduction fluid loop be maximized. As such, the diameter (referred to as the “caliper”) of the wells must be strictly controlled so that cavitating (also known as washout) of the well bore does not occur. Washout of the well bore caliper, or an unnecessary increase in the diameter of the well bore, results in a loss of the loop's ability to transfer a considerable percentage of energy. Large voids or large caliper well bores require much more annular space to be filled between the loop and the bore hole wall with grout material. This results in a loss of thermal conductance from the earth to the loop at that point.
Air Hammer Drilling
A myriad of drilling techniques are available for geothermal well production, including air hammer drilling (which is typically utilized in consolidated, e.g. Bed Rock formations), and circulating mud drilling, (which is utilized in glacial drift or overburden e.g., gravel, sand, and clay).
Air hammer drilling utilizes a rotary bit that slams against, then removes bits of, the consolidated formation being drilled. Air rotary drilling methods are almost exclusively utilized in hard consolidated formations to speed up and cut costs of drilling in bedrock. Air compressors are utilized to force air down the drill pipe through a down hole air hammer on the bottom end of the drill string. Exhaust air from the hammer evacuates the area between the drill string and the wall of the bore hole thereby lifting large volumes of water and mud out of the bore hole.
Air rotary drilling cuts through dense structures (i.e. bedrock layers) quickly, and, compared to circulating mud drilling, it is particularly useful when lost circulation occurs. This is because the air used in air rotary drilling technique tends to lift water, which seeps into the well bore (from fissures, aquifers and other voids).
Environmental containment of the drill site with air rotary drilling is very challenging. Fuel consumption of equipment utilizing this method is extremely high due to massive amounts of horse power spent producing huge amounts of air at extremely high pressures. A 6 inch diameter bore hole at five hundred ft in depth requires constant generation of up to 1000 CFM (cubic feet per minute) at 350 psi (pounds per square inch). This is twice the horse power required by mud rotary systems to drill at the same depth.
Other drawbacks to air rotary drilling include disruption of adjacent structures such as aquifers and nearby wells. As such, air hammer drilling is best utilized when wells are spaced at least 150 feet from each other.
Mud Rotary Drilling
Mud rotary drilling uses mud to carry away cuttings. FIG. 2A is a schematic of a standard drill-string 12 with mud rotary drilling in use. The down-pointing and up-pointing arrows show the direction of drilling mud, which is initially injected at the top center of the drill string. The drilling mud is pumped through the center of the drill string and out of the rotary bit 14. With continued pumping, the mud is pushed to the surface of the well bore, taking with it the bore cuttings entrained in the mud. Thus, the mud serves as a vehicle to remove bore cuttings as they are produced.
Mud rotary drilling is less disruptive to nearby geologic structures, but also less effective in penetrating dense structures even when expensive diamond bits (such as those featuring polycrystalline diamond compact (PDC) inserts) are used.
Also, mud rotary drilling stops working when large cavities develop or are encountered during drilling, inasmuch as mud pressure drops significantly in these scenarios. A subsequent drop in the return mud volume through the annulus (i.e., the space between the drill string and the sides of the well bore) results in cuttings not being carried to the surface of the hole for evacuation. This reduction of flow may generally be classified as seepage (less than 20 bbl/hr [3 m3/hr]), partial lost returns (greater than 20 bbl/hr [3 m3/hr] but still some returns), and total lost returns (where no fluid comes out of the annulus). In this severe latter case, the hole may not remain full of fluid even if the pumps are turned off. If the hole does not remain full of fluid, the vertical height of the fluid column is reduced and the pressure exerted on the open formations is reduced. This in turn can result in another zone flowing into the wellbore and a catastrophic loss of well control.
Contained mud rotary systems provide a reserve capacity for generating more mud. But, such systems usually cannot generate enough mud to overcome the aforementioned pressure and/or volume drop when large cavities are encountered in consolidated formations. At that point, the mud rotary drilling is finished, and other drilling methods must be applied.
In light of the foregoing, state of the art geothermal field development relegates the use of geothermal energy to venues able to accommodate large silt ponds, high volume water run off, and substantial scarring of the landscape associated with air hammer drilling. As such, large campuses, outlying industrial sites, or abandoned brown fields heretofore were the only candidates for geothermal well development.
Current industry standards set by The International Ground Source Heat Pump Association (IGSHPA) specifies grid pattern spacing of 10 ft to 20 ft between wells. Often, geothermal wells are 150 ft to 200 ft in depth depending on the relationship and distance from the equator. Each of these wells yield approximately one ton or 12,000 BTU of geothermal energy. Most single family homes are approximately 2000 square feet of living space. Modern built homes require from 3 to 4 ton of geothermal energy to supply heat pump load requirements. Three to four wells spaced 20 feet apartment usually can be accomplished in most rural back yards; however the much larger tonnage requirements of high rise buildings and commercial businesses make the possibility of installing geothermal well fields on sidewalks, alley ways, and parking lots a real challenge. Given that most commercial loads are a minimum of 20 to 30 tons, and therefore require a minimum of 20-30 wells, such a geothermal well field typically requires 200 to 300 foot blocks of space.
Drilling deeper wells has not been an attractive option for multiple reasons:
1. The geothermal well drilling industry has no method for assuring a consistent caliper for wells at any depth.
2. Deep well drilling results in massive amounts of water and drilling spoils (cuttings) generated during air rotary drilling. This raises environmental issues.
3. Lack of a method for competitively using mud rotary drilling in consolidated formations. Loss of mud circulation becomes particularly acute in deep drilling. State of the art mud rotary drilling methods are not effective after lost circulation zones are encountered; therefore casings must be set deep through the zone. This casing installation is neither cost effective nor easy to remove.
4. Deterioration of silica sand-based grout during air rotary drilling in deep consolidated formations. This leads to contamination of fresh water aquifers.
A need exists in the art for a system and a method for applying geothermal energy to footprints not exceeding a standard city lot. The system and method should accommodate field development on the city lot already containing a house, an ongoing commercial enterprise, or other permanent structure. The system and method should also obviate the need for completely retrofitting the HVAC of the permanent structure to utilize the geothermal energy. The method should also optimize state of the art mud rotary drilling techniques for their use in lost circulation zones.